The present invention relates to switch input circuits. In particular, the present invention relates to a switch input circuit having a power-saving device, for example during an application of a wetting current to the switch or switches.
Automotive switching systems that are connected to electronic control units may require a certain current flow when the switch contacts are closed, in order to xe2x80x98cleanxe2x80x99 the contacts of any oxidation or other contaminants. This current may be referred to as the wetting current, and may be defined with reference to a,particular voltage, for example  greater than 10 mA at 12 volts.
An approach may be to simply provide a pull-up or pull-down resistor associated with the input processing circuitry in the control unit. This pull-up resistor may be driven by a transistor so that the wetting current may be switched on or off by a control signal connected to the base of the transistor, thereby reducing quiescent current flow.
When the switch contacts are closed, power may be dissipated by the pull-up resistor in the form of heat. Therefore, a suitable resistor may be required to be chosen which may dissipate this heat under the worst case conditions, for example at maximum battery voltage and maximum operating temperature. Depending on the application, for example if the circuit is located in a confined space and there are many switch inputs, the heat generated may cause problems with other electrical components. The problems with power dissipation may become even worse in truck systems having 24 volt batteries, because power may be proportional to voltage.
However, it may also be desirable to keep the wetting current at a relatively high level over the contact cleaning period, in order to effectively clean the switch or switches.
For example: for a 24 volt supply for a truck switch input circuit, a resistor of 1800 Ohms may be required to provide 10 mA at 18V, and may dissipate 320 mW at 24 volts. At the maximum 32 volts, this resistor may dissipate 570 mW.
The present invention may provide an exemplary method of providing a wetting current to at least one switch through a respective resistor, characterized by modulating the wetting current to reduce average power consumption of the respective resistive element.
The pulse width modulation signal may be supplied to the base of a transistor to periodically allow the wetting current to flow through the emitter and collector of the transistor into the switch input circuit, in accordance with the duty cycle of the pulse width modulation signal.
The method may further include the step of sensing the number of closed switches connected to the switch input circuit. The method may further include the step of providing adjustment of the pulse width modulation signal in response to the sensed number of closed switches. The step of providing adjustment may include increasing the duty cycle of the pulse width modulation signal, if the sensed number of closed switches increases.
The method may further include the step of determining the voltage level of a voltage supply of the circuit. The step of determining may include sensing the voltage level using an analog-to-digital converter to thereby determine a digital value representative of the voltage level. The method may further include the steps of: determining, from the digital value, which of a plurality of predetermined voltage ranges the voltage level of the voltage supply falls within; and adjusting the duty cycle of the pulse width modulation signal depending on the relevant voltage range of the voltage supply.
The present invention may further provide a switch input circuit having a current source for providing the wetting current to at least one switch through a respective resistive element, characterized by a modulation arrangement for modulating the wetting current to provide a reduced average power consumption of the respective resistive element.
The present invention may further provide a switch input circuit having improved power consumption characteristics, the circuit including a current source for supplying a wetting current to at least one switch, and a pulse width modulation signal for modulating the supply of the wetting current to the at least one switch to thereby reduce the average wetting current thus supplied.
The present invention may further provide a method of improving power consumption characteristics of a switch input circuit, including the steps of: providing a wetting current to at least one switch; modulating the wetting current with a pulse width modulation signal to reduce the average wetting current provided to the at least one switch.
Exemplary embodiments of the present invention may be implemented without additional hardware, provided that the filter capacitors used on the inputs are sufficient to ensure electromagnetic compatibility (EMC), and that the microcontroller delivers the appropriate pulse width modulation (PWM) signal.